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Ballistics pendulum on steroids

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Vb1702
#1
Dec26-09, 08:52 AM
P: 1
1. The problem statement, all variables and given/known data

I am doing a science fair experiment using a ballistics pendulum. Instead of a wooden block to fire into I am using ballistics gelatin. This will allow me to measure the amount of energy deposited into the gelatin. A ballistics pendulum is usually used to determine velocity but I want to determine the amount of energy. I can determine velocity using available data. Most of the data on ballistics pendulums indicates the collision is inelastic. I assume this is because all of the energy is deposited into the wooden block. I believe in this instance the collision will be elastic, not perfectly elastic, since the two objects will move independently after the collision. The gelatin will initially move with the bullet but then the bullet will continue on and the gelatin will return to its original position.

2. Relevant equations

KEi= 1/2m vo^2

where m = mass of bullet and vo = the initial velocity of the bullet

KEf = 1/2 (m+M) vf^2

where M = mass of pendulum and vf = velocity of pendulum and ball

PEf = (m+M)(g)(h)

where g = gravitational acceleration
and h = change in height of gel block

3. The attempt at a solution

It seems like I should be able to just use the equation PEf = (m+M)(g)(h) to determine the kinetic energy converted to potential energy in the gel block but the more I read the less sure I am about this. I don't have any data yet as I am just in the research stage. I just need to determine if I'm right about this being an elastic collision and using the PEf equation to determine the amount of energy deposited into the gel.

Maybe, since kinetic energy is being converted to internal energy (in the gel block) and potential energy I can't determine the amount of energy deposited? Maybe, this is conservation of momentum and not conservation of energy? Maybe I'm in over my head!

Help!!!

Thanks for your help.
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pgardn
#2
Dec26-09, 12:02 PM
P: 621
Seems a bit experimentally complicated as ballistic pendulums usually are used for perfectly inelastic collisions.

I have a suggestion:

How about firing your projectile off a perfectly horizontal table against a wall marked with a line (height of the table). If you know the height of the table and you know how far down the wall the projectile dropped if it makes an imprint on the wall you can determine the velocity of the projectile. You can measure delta x (distance from the table to the wall) and you know delta y (the distance the projectile dropped while in the air, the diffrence between the height of the table and how far down the wall the projectile hit). The delta x gives you horizontal distance and the delta y can yield the time the projectile is in the air. This allows you to calculate V initial and its all Vx cause you shot the projectile horizontally.

Repeat the same experiment through the gel. The projectile should hit further down on the wall which means its spent more time in the air which means it was going slower giving you a diff V initial.

Then using the two diff. V's calculate the kinetic energy and the diff between the two which would mostly be the loss of mechanical energy while traveling through the gel.

I think this would be lots easier.


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